This invention relates to a light transmission device for use in transmitting an output light beam by subjecting a sequence of digital signals to predetermined modulation, such as frequency shift keying, phase shift keying.
In a conventional light transmission device of the type described, a coherent light beam is modulated by a sequence of digital signals to be sent as an output light beam to an optical fiber. Such modulation may be, for example, frequency shift keying, phase shift keying. In any event, the light transmission device is excellent in view of a high speed modulation characteristic and a long distance transmission characteristic. In addition, the light transmission device can accomplish a very large capacity of communication. Under the circumstances, the light transmission device is therefore greatly expected as a next generation device and is considered about a wide spread use.
Herein, it is to be noted that the coherent light beam is modulated by the digital signal sequence with a modulation index.
In order to correctly receive the output light beam transmitted from the light transmission device, optical coherent detection or optical heterodyne detection is executed by a light reception device. To this end, the modulation index should not be varied with time but should be kept stable by the light transmission device. Thus, it is of importance to keep the modulation index stable in the light transmission device.
However, it is practically difficult either to stabilize or set the modulation index in the light transmission device or to stably control the modulation index.